Phosgene manufacturing process

ABSTRACT

A process for producing phosgene is disclosed which involves contacting a mixture comprising carbon monoxide and chlorine (e.g., at about 300° C. or less) with carbon which (1) has a micropore to macropore ratio of 3.5 or less; (2) has a high degree of oxidative stability (i.e., loses about 16% of its weight, or less, in the WVC Temperature Test as defined herein); and (3) has a minimum surface area of at least 10 m 2  /g. The use of this carbon having an active metal content of 1000 ppm or more is disclosed.

This application is the national filing under 35 USC 371 ofInternational Application No. PCT/US97/22903 filed Dec. 15, 1997 andclaims benefit of Provisional Application Ser. No. 60/033,447 filed Dec.20, 1996.

FIELD OF THE INVENTION

This invention relates to a process for the manufacture of phosgene bythe reaction of chlorine with carbon monoxide in the presence of acarbon catalyst. More particularly, this invention relates to a processfor the manufacture of phosgene with minimal production of the hazardouschemical, carbon tetrachloride.

BACKGROUND

The production of phosgene by the reaction of chlorine with carbonmonoxide in the presence of a carbon catalyst is a well known process.The phosgene produced by this process will typically contain 400 to 500ppm by weight carbon tetrachloride. This amount, evaluated on the basisof the total world-wide production of phosgene of about ten billionpounds (4.5×10⁹ kg) corresponds to co-production of about 4 to 5 millionpounds (1.8×10⁶ kg to 2.3×10⁶ kg) of carbon tetrachloride with thephosgene.

Japanese patent publication (Kokoku) No. Hei 6[1994]-29129 disclosesthat the amount of carbon tetrachloride produced during the phosgenemanufacturing process can be reduced (e.g., by about 50%) by using anactivated carbon which has been washed with an acid and which contains atotal of 1.5 wt. % or less of metal components comprised of transitionmetals, boron, aluminum and silicon.

A process for producing phosgene using carbon having an active metalcontent of less than 1000 ppm and a weight loss of about 12% or less inthe WVC temperature test has been described (see U.S. application Ser.No. 60/012,021 and International Application No. PCT/US96/17526).

Carbon tetrachloride has been of concern in connection with ozonedepletion and global warming potentials. Therefore, there is an interestin developing phosgene processes in which the amount of carbontetrachloride impurity is minimized.

SUMMARY OF THE INVENTION

A process for producing phosgene is provided which comprises contactinga mixture comprising carbon monoxide and chlorine with carbon. Inaccordance with this invention the carbon (1) has a micropore tomacropore ratio of 3.5 or less; (2) loses about 16% of its weight, orless, when sequentially heated in air for the following times andtemperatures; 125° C. for 30 minutes. 200° C. for 30 minutes, 300° C.for 30 minutes, 350° C. for 45 minutes, 400° C. for 45 minutes, 450° C.for 45 minutes and finally at 500° C. for 30 minutes; and (3) has asurface area of at least 10 m² /g. Also, in accordance with thisinvention, the active metal content of the carbon may be 1000 ppm ormore. Typically the contact is at a temperature of about 300° C., orless.

DETAILED DESCRIPTION

The present invention relates to improving the production of phosgeneproduced by contacting carbon monoxide and chlorine with carbon. Theimprovement can be employed in connection with any of those carbon-basedprocesses used commercially or described in the art (e.g., thoseprocesses disclosed in U.S. Pat. Nos. 4,231,959 and 4,764,308).

Phosgene is commercially manufactured by passing carbon monoxide andchlorine over activated carbon. The reaction is strongly exothermic andis usually done in multitubular reactors to more effectively control thereaction temperature. Carbon monoxide is typically added in at least astoichiometric amount (often in stoichiometric excess) to minimize thefree chlorine content of the phosgene product.

As used in connection with this invention, the term "active metals"means metals included in the group consisting of transition metals ofgroups 3 to 10, boron, aluminum and silicon.

The carbon materials useful as catalysts for this invention are porous(i.e., a surface area of at least 10 m² /g) and contain both microporesand macropores. As used in connection with this invention, the term"micropore" means a pore size of 20 Å (2 nm) or less and the term"macropore" means a pore size of greater than 20 Å (2 nm). The totalpore volume and the pore volume distribution can be determined bymercury porosimetry. The micropore volume (cc/g) is subtracted from thetotal pore volume (cc/g) to determine the macropore volume. The ratio ofmicropores to macropores is then easily calculated. The carbons used forthis process have a micropore to macropore ratio of less than 3.5,preferably 2.0 or less.

The carbons used for the process of this invention also exhibitsubstantial weight stability when heated in air. More particularly, whenheated in air at 125° C. for 30 minutes, followed by heating at 200° C.for 30 minutes, followed by heating at 300° C. for 30 minutes, followedby heating at 350° C. for 45 minutes, followed by heating at 400° C. for45 minutes, followed by heating at 450° C. for 45 minutes and finallyfollowed by heating at 500° C. for 30 minutes, the carbons employed forthe process of this invention lose about 16% of their weight, or less.This sequence of time and temperature conditions for evaluating theeffect of heating carbon samples in air is defined herein as the "AVCTemperature Test". The WVC Temperature Test may be run using thermalgravimetric analysis (TGA). Carbons which when subjected to the WVCTemperature Test lose about 16% of their weight, or less, are consideredto be advantageously oxidatively stable. Preferably the weight loss inthe WVC Temperature Test is 10% or less, more preferably 5% or less.

Low active metal content carbons can be used to achieve low carbontetrachloride formation (see International Application No.PCT/US96/17526). Low active metal carbons include three dimensionalmatrix porous carbonaceous materials. For example, the porouscarbonaceous materials of Examples C and D in International ApplicationNo. PCT/US96/17526 have been measured to have total pore volumes ofabout 0.58 and 0.90 cc/g, respectively, and micropore to macroporeratios of about zero. Three dimensional matrix porous carbonaceousmaterials are described in U.S. Pat. No. 4,978.649, which is herebyincorporated by reference herein in its entirety. Of note are threedimensional matrix carbonaceous materials which are obtained byintroducing gaseous or vaporous carbon-containing compounds (e.g.,hydrocarbons) into a mass of granules of a carbonaceous material (e.g.,carbon black); decomposing the carbon-containing compounds to depositcarbon on the surface of the granules; and treating the resultingmaterial with an activator gas comprising steam to provide a porouscarbonaceous material. A carbon-carbon composite material is thusformed.

We have, however, discovered that when the above described micropore tomacropore ratios and weight stability criteria are satisfied, then thecarbon can contain 1000 ppm or greater by weight of active metals. Moresuprisingly, even the iron content can be greater than 1000 ppm byweight. Iron is known to accelerate carbon tetrachloride formation. Ofnote are embodiments where active metals are 2000 ppm or more.

Carbon from any of the following sources arc useful for the process ofthis invention; wood, peat, coal, coconut shells, bones, lignite,petroleum-based residues and sugar; provided that they are treated, ifnecessary, to reduce the micropore volume. Commercially availablecarbons which may be used in this invention include those sold under thefollowing trademarks: Calgon X-BCP and Calsicat. The carbon support canbe in the form of powder, granules, or pellets, or the like.

The carbon surface area as determined by BET measurement is preferablygreater than about 100 m² /g and more preferably greater than about 300m² /g (e.g., from 550 to 1000 m² /g). Typically, surface areas are 2000m² /g or less.

It is known from dissociation equilibria that at 100° C., phosgenecontains about 50 ppm chlorine; and that at 200° C., about 0.4%, at 300°C., about 5% and at 400° C. about 20% of the phosgene is dissociatedinto carbon monoxide and chlorine. Also, the higher the reactiontemperature, the more carbon tetrachloride is generally produced.Accordingly, the temperature of the reaction is generally stable.Preferably the weight loss in the WVC Temperature Test is 10% or less,more preferably 5% or less.

Low active metal content carbons can be used to achieve low carbontetrachloride formation (see International Application No.PCT/US96/17526). Low active metal carbons include three dimensionalmatrix porous carbonaceous materials. For example, the porouscarbonaceous materials of Examples C and D in International ApplicationNo. PCT/US96/17526 have been measured to have total pore volumes ofabout 0.58 and 0.90 cc/g, respectively, and micropore to macroporeratios of about zero. Three dimensional matrix porous carbonaceousmaterials are described in U.S. Pat. No. 4,978,649, which is herebyincorporated by reference herein in its entirety. Of note are threedimensional matrix carbonaceous materials which are obtained byintroducing gaseous or vaporous carbon-containing compounds (e.g.,hydrocarbons) into a mass of granules of a carbonaceous material (e.g.,carbon black); decomposing the carbon-containing compounds to depositcarbon on the surface of the granules; and treating the resultingmaterial with an activator gas comprising steam to provide a porouscarbonaceous material. A carbon-carbon composite material is thusformed.

We have, however, discovered that when the above described micropore tomacropore ratios and weight stability criteria are satisfied, then thecarbon can contain 1000 ppm or greater by weight of active metals. Moresuprisingly, even the iron content can be greater than 1000 ppm byweight. Iron is known to accelerate carbon tetrachloride formation. Ofnote are embodiments where active metals are 2000 ppm or more.

Carbon from any of the following sources are useful for the process ofthis invention; wood, peat, coal, coconut shells, bones, lignite,petroleum-based residues and sugar; provided that they are treated, ifnecessary, to reduce the micropore volume. Commercially availablecarbons which may be used in this invention include those sold under thefollowing trademarks: Calgon X-BCP and Calsicat. The carbon support canbe in the form of powder, granules, or pellets, or the like.

The carbon surface area as determined by BET measurement is preferablygreater than about 100 m² /g and more preferably greater than about 300m² /g (e.g., from 550 to 1000 m² /g). Typically, surface areas are 2000m² /g or less.

It is known from dissociation equilibria that at 100° C., phosgenecontains about 50 ppm chlorine; and that at 200° C., about 0.4%, at 300°C., about 5% and at 400° C. about 20% of the phosgene is dissociatedinto carbon monoxide and chlorine. Also, the higher the reactiontemperature, the more carbon tetrachloride is generally produced.Accordingly, the temperature of the reaction is generally about 300° C.,or less (e.g., in the range of from 40° C. to 300° C.). Preferably, thetemperature of the process is from about 50° C. to 200° C.; morepreferably from about 50° C. to 150° C. The phosgene produced by theprocess of this invention typically contains about 350 ppm by weight orless of carbon tetrachloride, based upon phosgene (i.e., 350 parts byweight CCl₄ per million parts by weight COCl₂, or less) even at atemperature of 300° C. Preferably, the reaction temperature and thecarbon are chosen to provide phosgene which contains less than about 250ppm by weight of carbon tetrachloride; and more preferably, are chosento provide phosgene which contains less than about 100 ppm by weight ofcarbon tetrachloride, based upon phosgene. Of note are embodiments wherethe reaction time and temperature are controlled to provide a carbontetrachloride concentration of about 100 ppm or less based upon thetotal product stream.

Without further elaboration, it is believed that one skilled in the artcan, using the description herein, utilize the present invention to itsfullest extent. The following specific embodiments are, therefore, to beconstrued as merely illustrative, and does not constrain the remainderof the disclosure in any way whatsoever.

EXAMPLES

General Catalyst Testing Procedure

A 1/2" (12.7 mm) O.D.×15" (381 mm) Inconel™ 600 nickel alloy tubecontaining a 100 mesh (0.015 mm) Monel™ nickel alloy screen was used asthe reactor. The reactor was charged with about 2.5 mL to about 8 mL ofcarbon catalyst and heated to 300° C. This was the temperature used forall the examples.

A 1:1 molar ratio mixture of carbon monoxide and chlorine was passedover the catalyst. The contact times were between 8 to 12 seconds. Theexperimental results are shown in Table 1.

The comparative examples were done in the same way as described above.The results are shown in Table A.

General Analytical Procedure

The reactor effluent was sampled on-line with a Hewlett Packard HP 5890gas chromatograph using a 105 m long, 0.25 mm I.D. column containingRestak™ RTX-1 Crossbond 100% dimethyl polysiloxane. Gas chromatographicconditions were 50° C. for 10 minutes followed by temperatureprogramming to 200° C. at a rate of 15° C./minute. The smallest amountof carbon tetrachloride that could be quantitatively identified by gaschromatography was about 40 ppm by weight. For greater sensitivity an online mass spectrometer detector was used and calibrated to determineconcentrations of less than 40 ppm.

The BET surface area, pore volumes and pore distributions were obtainedusing a Micromeritics ASAP 2400 instrument.

Thermal Analysis Procedure Thermal gravimetric analysis (TGA) was doneusing a TA Instruments analyzer. The TGA experiments were done in air ata flow rate of 80 mL/min. The carbon sample was heated in air for thefollowing times and temperatures; 125° C. for 30 minutes, 200° C. for 30minutes, 300° C. for 30 minutes, 350° C. for 45 minutes, 400° C. for 45minutes, 450° C. for 45 minutes and finally at 500° C. for 30 minutes.The weight loss was measured at each interval and finally aftercompletion of the heating cycle. The percentage weight loss aftercompletion of the heating cycle at 500° C. is shown in the tables.

Legend

Carbon Sample

A. High surface area coconut shell carbon, Calsicat

B. Partially graphitized carbon, Englehard

C. 1/16" (1.6 mm) graphite carbon, Englehard

D. Type X-BCP proprietary carbon, Calgon Carbon Corp.

R. GRC-11 proprietary carbon, Calgon Carbon Corp.

S. Centaur proprietary carbon, Calgon Carbon Corp.

T. Coconut shell carbon type 507, Barnebey & Sutcliffe Corp.

                                      TABLE 1                                     __________________________________________________________________________                 TGA Active          Total                                                                              Micro-                                                                             Micro                                  CCl.sub.4 Wt. Metal Fe Surface Pore Pore Macro                               Carbon Conc..sup.1 Loss.sup.2 Content.sup.3 Content Area Volume Volume                                                Pore                                 Ex. Sample ppm wt. % ppm ppm m.sup.2 /g cc/g cc/g Ratio                     __________________________________________________________________________    1  A    330  15.39                                                                             2050   153 1661 0.81 0.61 3.1                                  2 B <12  4.31 4580 3000  836 0.44 0.26 1.4                                    3 C  <8  2.18 2470 1000  633 0.39 0   0                                       4 D  90 11.19 13,300   1900 1279 0.64 0.41 1.8                              __________________________________________________________________________     .sup.1 By weight as ppm of the COCl.sub.2 product. The values shown are       averages taken over 7 hours and are highend estimates                         .sup.2 The carbon sample was heated in air for the following times and        temperatures: 125° C. for 30 minutes, 200° C. for 30            minutes, 300° C. for 30 minutes, 350° C. for 45 minutes,        400° C. for 45 minutes, 450° C. for 45 minutes and finally      at 500° C. for 30 minutes. The wt. loss recorded occurred between      125 and 500° C.                                                        .sup.3 Active metals consist of transition metals of groups 3 to 10,          boron, aluminum and silicon                                              

COMPARATIVE EXAMPLES

                                      TABLE A                                     __________________________________________________________________________                 TGA Active          Total                                                                              Micro-                                                                             Micro                                  CCl.sub.4 Wt. Metal Fe Surface Pore Pore Macro                               Carbon Conc..sup.1 Loss.sup.2 Content.sup.3 Content Area Volume Volume                                                Pore                                 Ex. Sample ppm wt. % ppm ppm m.sup.2 /g cc/g cc/g Ratio                     __________________________________________________________________________    A  R    480  88.2                                                                              1520  130  835  0.40 0.36 9.0                                  B S 790 59.9 21,100   1900  708 0.35 0.30 6.0                                 C T 490 89.8 4900 360 1012  0.50 0.43 6.1                                   __________________________________________________________________________     .sup.1 By weight as ppm of the COCl.sub.2 product. The values shown are       averages taken over 7 hours and are highend estimates                         .sup.2 The carbon sample was heated in air for the following times and        temperatures: 125° C. for 30 minutes, 200° C. for 30            minutes, 300° C. for 30 minutes, 350° C. for 45 minutes,        400° C. for 45 minutes, 450° C. for 45 minutes and finally      at 500° C. for 30 minutes. The wt. loss recorded occurred between      125 and 500° C.                                                        .sup.3 Active metals consist of transition metals of groups 3 to 10,          boron, aluminum and silicon                                              

What is claimed is:
 1. A process for producing phosgene, comprisingcontacting a mixture comprising carbon monoxide and chlorine with carbonwhich (1) has a micropore to macropore ratio of 3.5 or less; (2) losesabout 16% of its weight, or less, in the WVC Temperature Test; (3) has asurface area of at least 10 m² /g; and (4) has an active metal contentof 1000 ppm or more.
 2. The process of claim 1 where the contact is at atemperature of about 300° C. or less.
 3. The process of claim 2 whereinthe active metal content of the carbon is 2000 ppm or more.
 4. Theprocess of claim 3 wherein the carbon surface area is greater than about100 m² /g.
 5. The process of claim 4 wherein the micropore to macroporeratio in the carbon is 2.0, or less.
 6. The process of claim 5 whereinthe carbon loses about 5% of its weight or less in the WVC TemperatureTest.